Information is typically communicated between a ground station and an aircraft using a VHF communications apparatus that operates in a frequency band allocated to aircraft operation. For example, voice communications between aircraft and ground-based air traffic control facilities are generally carried out by means of a VHF communications transceiver that operates in a frequency band from 118.0 MHz to 135.95 MHz on any of a plurality of selectable and discrete channel frequencies in this band. Data may also be communicated between a ground station and an aircraft by coupling a modem to the VHF communications transceiver so that data may be communicated using audio frequency tones. For example, the well-known Aircraft Communications Addressing and Reporting System (ACARS) and the Aeronautical Telecommunications Network (ATN) provide a data link capability so that character-oriented and bit-oriented communications may occur between the ground station and the aircraft.
Since radio communications at VHF frequencies is generally limited to line of sight propagation, communications between the ground station and the aircraft are generally not possible after the aircraft has flown beyond the horizon. If intervening geographical obstructions, such as a mountain range, are present between the ground station and the aircraft, communications may not be possible even though the aircraft has not yet flown beyond the horizon. In order to maintain continuous communications between a ground station and an aircraft, a plurality of intervening ground stations are generally required to relay signals between a central station originating the communications and the aircraft as it proceeds along a given flight route. Accordingly, in a circuit-switched radio system, a connection is created between the originating ground station and the aircraft through one or more relay stations by dedicating a predetermined amount of transmission capacity to the connection. Alternately, in a packet-switched system (such as ACARS or ATN), a connection is created between the originating station and the aircraft by transmitting data in packets having address and control data encoded on discrete portions of the communication. As a result, several connections may use the same transmission path simultaneously, since the path is dedicated to a single connection only for the packet transmission.
In either case, a communications path from the ground station to the aircraft must be selected and enabled. Signal routing may be based upon prior calculation, or upon operational experience obtained from aircraft that regularly navigate along a selected route. For example, when selecting a ground station, logic associated with the communications system in the aircraft may be configured so that the communications system operates as if the system is positioned at or above a reference altitude (typically about twenty-five thousand feet) even though the aircraft may be positioned at an altitude that is below the reference altitude. While this assumption generally permits the selection of an appropriate ground station in most instances, when the aircraft is operated at an altitude that is significantly below the reference altitude, such as on the ground, in a holding pattern or in the approach structure, communication difficulties may arise. Thus, the “hand-over” point, which is a location where the aircraft ceases communicating with one ground station, and initiates communications with another, may not be reliably determined in communication systems so configured.
In other known communications systems, the logic associated with the system is configured to minimize the number of ground stations selected as the aircraft navigates along a route, since the overall cost of air-ground communications is at least in part determined by the number of ground station hand-overs that occur along the route. This approach is nevertheless subject to certain operational disadvantages. For example, the system may continue communicating with a selected ground station even though there are ground stations that are positioned closer to the aircraft, which may provide relatively higher signal strengths, and therefore provide more reliable communications between the aircraft and the ground station.
What is needed is a communications system that is dynamically configurable so that ground station selection is not dependent upon a fixed aircraft altitude or upon the minimization of station hand-overs.